A solar cell converts sunlight directly into electricity through the photovoltaic effect. It is constructed from layers of semiconductor materials, usually silicon, with a positive and negative type. When light photons are absorbed, they create electron-hole pairs which generate an electric current across the positive-negative junction. Solar cells can be connected together in solar panels or modules to increase the voltage and power output for practical applications. The most efficient solar cells are made from monocrystalline silicon but polycrystalline and amorphous silicon types are also used.

1. What is a solar cell?
• A structure that converts solar energy directly to
DC electric energy.
– It supplies a voltage and a current to a resistive load
(light, battery, motor).
• It is like a battery because it supplies DC power.
• It is different from a battery in the sense that the
voltage supplied by the cell changes with changes
in the resistance of the load.

2. Basic Physics of Solar Cells
• Silicon (Si) is from group 4 of the period
table. When many Si atoms are in close
proximity, the energy states form bands of
forbidden energy states.
• One of these bands is called the band
gap(Eg) and the absorption of light in Si is a
strong function of Eg.

3. Basic Physics of Solar Cells
• Si is covalently bonded: It shares electrons.
– When a Si atom is replaced with a group 3 (Al, B) it
forms a positive particle called a hole that can move
around the crystal through diffusion or drift (electric
field).
– When a Si atom is replaced with a group 5 (As, P) it
forms an electron that can move around the crystal.
– By selectively doping the Si Crystal when can change
the resistivity and which type of carrier transfers charge
(carries current). Because we can selectively dope a Si
crystal it is called a semiconductor.

4. Photovoltaic effect
Definition:
The generation
of voltage across the
PN junction in a
semiconductor due
to the absorption of
light radiation is
called photovoltaic
effect. The Devices
based on this effect
is called photovoltaic
device.
Light
energy
n-type semiconductor
p- type semiconductor
Electrical
Power
p-n junction

5. Basics of solar cells
• If two differently contaminated semiconductor layers are
combined, then a so-called p-n-junction results on the boundary
of the layers.
• By doping trivalent element, we get p-type semiconductor. (with
excess amount of hole)
• By doping pentavalent element, we get n-type semiconductor
( with excess amount of electron)
n-type semiconductor
p- type semiconductor
p-n junction layer

6. Solar Cell Principle
 Operating diode in fourth quadrant generates power

7. Solar Energy Spectrum
•
• Power reaching earth 1.37 KW/m2

8. Electron Hole Formation
• Photovoltaic energy conversion relies on the number
of photons striking the earth. (photon is a flux of light
particles)
• On a clear day, about 4.4 x 1017
photons strike a square
centimeter of the Earth's surface every second.
• Only some of these photons - those with energy in
excess of the band gap - can be converted into
electricity by the solar cell.
• When such photon enters the semiconductor, it may
be absorbed and promote an electron from the valence band
to the conduction band.

9. • Therefore, a vacant is created in the valence band and it is
called hole.
• Now, the electron in the conduction band and hole in
valence band combine together and forms electron-hole pairs.
hole
Valence band
Conduction band
electron
Photons

10. A solar panel (or) Solar array
Single solar cell
• The single solar cell constitute the n-type layer
sandwiched with p-type layer.
• The most commonly known solar cell is configured as a
large-area p-n junction made from silicon wafer.
• A single cell can produce only very tiny amounts of electricity
• It can be used only to light up a small light bulb or power a
calculator.
• Single photovoltaic cells are used in many small electronic
appliances such as watches and calculators

11. N-type
P-type
Single Solar cell

12. Solar panel (or) solar array (or) Solar module
The solar panel (or) solar array is the interconnection of
number of solar module to get efficient power.
• A solar module consists of number of interconnected
solar cells.
• These interconnected cells embedded between two
glass plate to protect from the bad whether.
• Since absorption area of module is high, more energy
can be produced.

14. Based on the types of crystal used, soar cells can be classified as,
1. Monocrystalline silicon cells
2. Polycrystalline silicon cells
3. Amorphous silicon cells
1. The Monocrystalline silicon cell is produced from
pure silicon (single crystal). Since the Monocrystalline
silicon is pure and defect free, the efficiency of cell will be
higher.
2. In polycrystalline solar cell, liquid silicon is used as raw material
and polycrystalline silicon was obtained followed by solidification
process. The materials contain various crystalline sizes. Hence,
the efficiency of this type of cell is less than Monocrystalline cell.
Types of Solar cell

15. Amorphous silicon is obtained by depositing silicon film
on the substrate like glass plate.
•The layer thickness amounts to less than 1µm – the
thickness of a human hair for comparison is 50-100 µm.
•The efficiency of amorphous cells is much lower than that
of the other two cell types.
• As a result, they are used mainly in low power
equipment, such as watches and pocket calculators,
or as facade elements.
Amorphous Silicon

16. Comparison of Types of solar cell
Material Efficiency (%)
Monocrystalline silicon 14-17
Polycrystalline silicon 13-15
Amorphous silicon 5-7

22. • There are two currents in a solar cell
1. Current due to reverse biased junction (IS) (Diode Current)
2. Current due to photovoltaic effect (IL) [Also called reverse
current]
3. The two currents are in opposite directions